Quantitative Traits and Heritability

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Transcript of Quantitative Traits and Heritability

Quantitative Traits and HeritabilityWhat is the difference ?Geoff Morrismorrisg@uvic.ca can be explained in Mendelian terms by many genes, each behaving individually in Mendelian fashion, contributing to the phenotype in a cumulative or quantitative way

Quantitative traitsFigure 23.2Controlled by two genes

Each gene has an additive allele (A) and a nonadditive allele (a)A/BRed=Figure 23.3n = number of genes involved

1/4 = number of F2 progeny expressing either extreme phenotypenExample: A cross is made between a red flowering and a white flowering plant. The F1 is a uniform pink, but in the F2, 1/64 of the plants have white flowers, 1/64 have red, and the rest have varying shades of pink to red. How many genes are estimated to be affecting the trait?a) 3b) 4c) 8Many polygenic traits are difficult or impossible to categorize into several discrete genotypic categories.

This is especially true when:The number of genes controlling the trait increasesThe influence of the environment increases12HeritabilityAll traits of biological organisms may be influenced by genetics and the environment.Determining the relative amount that genetics and environment control a quantitative trait is very difficult.Instead, focus on how variation in genetics and environment affect the phenotype.= part due to geneticsIf all phenotypic variation in a group is due to genetics,Heritability = 1

If all phenotypic variation is due to environment,Heritability = 0Rank these traits from most heritable, to least:Height

Heart Disease

Eye Colour

IQ

Type-2 Diabetes describes the amount of phenotypic due to genetic variation for a raised in a Heritabilityvariation particular populationparticular environmentTwin Studies Allow an Estimation of Heritability in HumansIdenticalFraternalV = V + V PGEConcordant: both twins express the trait, or neither does.Discordant: only one twin expresses the trait.Most meaningful to compare concordance between twin typesBlood TypeEye ColourMeaslesHandednessSchizophreniaCleft LipMammary Cancer10098958969426TraitMZConcordance Values Between Twin TypesQuantitative traits are often at an intermediate value in unselected populationsSelective Breeding of Species Can Alter Quantitative Traits DramaticallySelective breeding (artificial selection) is the modification of phenotypes in plants and animal species of economic importanceResults 80 generations laterDiffers from natural selection as it does not result from reproductive successGreyhoundGerman ShephardBulldogCharlieWild Mustard PlantStrainModified TraitKohlrabiStemKaleLeavesBroccoliFlower Buds and StemsBrussels SproutsLateral Leaf BudsCabbageTerminal Leaf BudCauliflowerFlower BudsVariation Underlies Selective BreedingStudy began in 1869 163 ears of corn with oil content of 4-6%19%< 1%Selective BreedingTwin StudiesQuantitative TraitsHeritabilityHowever, there is a limit81%Limitations?By the time they are born, MZ twins do not necessarily have identical genomes

Gene-expression patterns in MZ twins change with age, leading to phenotypic differences662887771053DZ+ V G x E1/64 is one of the extreme phenotype

1/4 = 1/64n=3n1/64 is one of the extreme phenotype

1/4 = 1/64n=3n34-53%

98%

60-70%

26% 2.

4.

1.

3.

5.V = V PEV = 1/2V + V PGEAt the start of the experiment the heritability for the trait is fairly high. At the end, it is near zero